Fix x11 poll_events to drain queue

It was only processing a single event per call. The docs say

> Fetches all the events that are pending, calls the callback function
> for each of them, and returns.

which suggests that was incorrect.

src/platform/linux/x11/mod.rs

@@ -111,16 +111,22 @@ impl EventsLoop {
         let xlib = &self.display.xlib;
 
         let mut xev = unsafe { mem::uninitialized() };
+        loop {
+            // Get next event
             unsafe {
                 // Ensure XNextEvent won't block
                 let count = (xlib.XPending)(self.display.display);
                 if count == 0 {
-                return;
+                    break;
                 }
 
                 (xlib.XNextEvent)(self.display.display, &mut xev);
             }
             self.process_event(&mut xev, &mut callback);
+            if self.interrupted.load(::std::sync::atomic::Ordering::Relaxed) {
+                break;
+            }
+        }
     }
 
     pub fn run_forever<F>(&self, mut callback: F)